Many users don't understand how EteSync works and think that this replaces
their existing apps. This should make it more obvious that they should
just use the apps they already use.
This is a monster commit because to be honest, it's a monster change. It
was impossible to do it in smaller steps because things just wouldn't
compile.
We couldn't do the migration step by step because they moved to Kotlin
which was causing a lot of troubles.
Now we are all on Kotlin, so things should hopefully work just fine.
It just needs a few tiny wrappers around some public statics. I'm not
doing that because it'll sort itself out the moment we update
vcard4android and ical4android.